An oil-free screw compressor in which lubricating oil (oil) is not supplied to between a screw of a male screw rotor and a screw of a female screw rotor, which engage with each other, has been used. In such an oil-free screw compressor, lubricating oil that has been supplied to bearings supporting shafts of the screw rotors is prevented from intruding into a rotor chamber accommodating the screws of the screw rotors. In particular, when a negative pressure is generated in the rotor chamber during an unload operation (an operation in a state where suction into the screw compressor is limited), the lubricating oil in the bearings is prevented from intruding into the rotor chamber.
For instance, an oil-free screw compressor described in JP S61-144289 U has a first shaft seal device and a second shaft seal device in a tubular shape that are fitted on a shaft of a screw rotor and are disposed between a screw of the screw rotor and a bearing. The first shaft seal device is disposed on the screw side of the screw rotor, and has a seal provided on the inner peripheral face of the first shaft seal device, and a communication portion provided on the bearing side with respect to the seal and communicating between the inner peripheral face side and the outer peripheral face side of the first shaft seal device. The second shaft seal device is disposed on the bearing side of the first shaft seal device, and has a seal provided on the inner peripheral face of the second shaft seal device, and a communication portion provided on the screw side of the screw rotor with respect to the seal and communicating between the inner peripheral face side and the outer peripheral face side of the second shaft seal device. The communication portion of the first shaft seal device communicates with the atmosphere outside of a casing accommodating the screw rotor via a first atmosphere communication portion formed in the casing. The communication portion of the second shaft seal device communicates with the atmosphere outside of the casing via a second atmosphere communication portion formed in the casing.
By a negative pressure generated in the rotor chamber during the unload operation, the atmosphere outside of the casing flows in through the communication portion of the first shaft seal device. However, when the atmosphere outside of the casing flows in only through the communication portion of the first shaft seal device, a negative pressure is generated in the seal on the inner peripheral face of the second shaft seal device, resulting in the possibility of intruding a small amount of lubricating oil into the rotor chamber. To eliminate the negative pressure generated in the seal for preventing intrusion of the lubricating oil, the atmosphere outside of the casing also flows in through the communication portion of the second shaft seal device.